docs(6209): test prose in the present tense — "used to", "previously", "before this" restated as the failure shape

Twenty-eight test files described the code they guard by what it did
before a fix. Each is now the property and the failure it prevents, in the
present tense: the backfill snapshot, the per-dial calibration query, the
rule scope, the lesson trigger filter, the subshell global, the trash owner
scope, the version readout fallback, and others. A docstring line that
narrated the backup suite's own history is removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_frontend_request_deadlines.py

@@ -14,7 +14,6 @@ resolves is indistinguishable from work still in progress. The practice names
     clients, a bare `await` on a stream — supplying the deadline is part of
     using it, not a hardening pass for later.
 
-Before this guard, no request in the app carried one.
 """
 from __future__ import annotations